Four-electrode system for the measurement of biological tissue conductivity at ELF and ULF

This article presents the metrological characterization of a four-electrode compact system able to measure the dielectric properties of biological tissues at extremely low and ultralow frequencies, where data available from the literature are very limited. The cell constant k of the system, together with its expanded uncertainty, is found measuring different saline solutions of known conductivity. Since the cell constant plays a key role in the determination of tissue dielectric properties, it has been further verified through tests on other saline solutions, containing a different type of solute, confirming the accuracy of the system. In particular, results on a saline solution with a given molar concentration of KCl and on a physiological solution (Eurospital 0.9% NaCl) show that the system maximum relative error is lower than 3.3%. Therefore, it can be concluded that the system correctly measures the conductivity in saline solutions, and the parameter k can be properly used for the measurement of dielectric properties of biological tissues. As an application example, the system is used to perform measurements on bovine liver. Liver conductivity measurements show a constant behavior as a function of frequency in the examined range. Furthermore, the comparison of our results with the few data found in the literature at low frequencies shows good agreement. These observations point out the feasibility and convenience of the proposed method for the measurement of the conductivity at very low frequency.